How To Develop Android Applications

how to develop android applications. Right now I am trying to write a very simple app that calculates miles per gallon. i have wrote two versions in regular JAVA but after the past few day I just have gotten stuck trying to teach myself how i can "port" it over to work on an android phone. This one is just works within the compiler

Code:
import java.util.Scanner;
public class Mpg
{
public static void main(String [] args)
{
[code]...

I'm trying to get a simple app set up like the 2nd version, but I'm having trouble setting up the boxes for user input and re displaying it once calculated.